SKIN
Objectives
You should be able to recognize the following in the
class slide sets:
| Thin skin | Meissner's corpuscles | |||
| Thick skin | Pacinian corpuscles | |||
| Epidermis | Eccrine sweat glands | |||
| stratum basalis (germinativum) | duct in epidermis | |||
| stratum spinosum | myoepithelial cells | |||
| stratum granulosum | Sebaceous glands | |||
| keratohyalin granules | Hair | |||
| stratum lucidum | hair follicle | |||
| stratum corneum | hair root (bulb) | |||
| keratinocytes | papilla | |||
| melanocytes | inner root sheath | |||
| melanin granules | outer root sheath | |||
| Langerhans cells (maybe) | arrector pili muscle | |||
| epidermal pegs | Hypodermis | |||
| Dermis | ||||
| dermal papillae | ||||
| papillary layer | ||||
| reticular layer | ||||
| * * * * * | ||||
 | Blue Histology |
Slides
| D-7 | Skin (elastic) | |
| D-161 | Thick skin (H&E) | |
| D-162 | Scalp (H&E) | |
| D-166 | Fingertip (silver) | |
| D-176 | External ear canal (H&E) |
Optional slides
| D-163 | Thin skin (H&E) | |
| D-164 | Toenail (H&E) | |
| D-165 | Fingertip (H&E) | |
| Thin skin also occurs on slides D-11, D-86, D-145, D-171 | |||
| Thick skin also occurs on slide D-145 | |||
Slide descriptions
D-161 Thick skin (H&E)
Hold this slide up and distinguish the bluish epidermis, pink layer of dermis 1-2 mm thick and subdermis ( = subcutaneous layer = hypodermis) composed mainly of adipose tissue and stained very palely (illustration). Scan the epidermis and dermis at low power. Note how they interlock with each other. Pegs of epidermis push down between the papillae of dermis. Realize that in three dimensions these pegs and papillae are not really mounds, but long ridges that are cut in cross-section. Find the irregular bottom edge of the dermis. The underlying subdermis is less compact, with many fat cells.
Study the epidermis in more detail with the 40X objective. Note the great changes in cell shape and chemical composition (revealed by staining properties) going from the basement membrane to the free surface. Identify the strata germinativum, spinosum, granulosum, lucidum and corneum, (illustration).The two lowest layers are sometimes referred to jointly as the Malpighian layer. These cells are still living and are called keratinocytes. Nearly all of the cell divisions occur in the single layer of basal cells that comprise the stratum germinativum. The most notable morphological features of the cells of the stratum spinosum are the vast number of desmosomes that firmly attach each cell to its neighbors. This allows skin to withstand strong abrasion. When the cells shrink during fixation their cell walls pull apart from one another, except where attached by the desmosomes. Thus, the cells seem to have spines connecting one to another, and are sometimes called "prickle cells". As the cells migrate into the stratum granulosum they accumulate droplets of keratohyalin in their cytoplasm. These droplets (usually referred to as keratohyalin granules, but are without a membrane around them) are strongly basophilic because the material is heavily phosphorylated. Abruptly these phosphate groups are removed and the droplets disperse. At the same time the nucleus and other organelles disappear. The dead cells form a pale featureless layer, the stratum lucidum. Sometimes this layer retains a faint bluish caste. The keratinization process continues to produce the very eosinophilic, insoluble band of keratin on the surface of the skin. The cells are so flattened and closely adherent to each other that cell boundries are indistinguishable unless artifactual cracks separate them. Eventually packets of keratinized cells are rubbed off of the surface.
Now, turn your attention to the dermis. Two zones can be distinguished. The papillary layer lies just under the epidermis, and includes the dermal papillae. It is composed of loose connective tissue with many cells. The thicker underlying reticular layer is composed of very dense irregular connective tissue, (illustration). Note the difference in types of blood vessels found in these two layers. What functions of the two layers account for this difference?
Find the sweat glands that you have already examined (illustration). There is nothing wrong with reviewing them again. These simple coiled glands have two parts, a duct portion and a secretory portion. The duct extends fairly straight from the epidermis down to the border with the hypodermis. There it coils around and then abruptly enlarges into the coiled secretory portion. The duct is made up of two layers of cuboidal epithelial cells that are relatively small and dark blue. The duct enters the epidermis at an epithelial peg and pursues a corkscrew path as a tubular hole among the epidermal cells (illustration).
The secreting portion is lined with a pseudostratified layer of plump cuboidal/pyramidal cells. These cells are lightly stained and have large prominent nuclei. Myoepithelial cells located between the secretory cells and the basal lamina help to squeeze the secreted sweat into the duct. They have the shapes of multi-armed starfish with their arms orientated along the tubular gland. In cross sections their arms are visible as red wedges at the outer edge of the gland. These are strongly eosinophilic due to an abundance of actin and myosin. Longitudinal or oblique sections show that the arms are elongate. Occasionally you will find their nuclei below the row of secretory cell nuclei. Myoepithelial cells, like the secretory cells, are ectodermal in origin and are part of the epithelium of the sweat gland.
Of course these sweat glands are of the eccrine type. Apocrine sweat glands have a limited distribution and you may recall that they are much larger than the more ubiquitous eccrine variety. You will not find apocrine glands in thick skin
This slide shows two types of sensory nerve endings; Meissner's touch corpuscles and Pacinian deep pressure corpuscles. If you have trouble finding them on this slide try D-165. If you still have problems see the course chairman for a replacement slide. Meissner's corpuscles (example 2) are located in the dermal papillae as close as possible to the surface of the skin. By being located directly under the epithelium, they are maximally sensitive to light touch. These barrel shaped structures consist of flat cells stacked transversely. A plexus of minute nerve endings invade between the stacked cells. These endings join to form a large, exiting, myelinated, sensory nerve fiber,
Much deeper in the skin, Pacinian
corpuscles resemble onions in cross-section. They average 2 mm in width and
4 mm in length. The outer part consists of some 50 lamellae, arranged loosely
and concentrically. The lamellae are made of thin flat endothelial-like cells,
connective tissue fibers and interstitial fluid. A large myelinated nerve fiber
pierces the corpuscle at one pole and continues through the core as a straight,
unbranched and unmyelinated nerve fiber. These mechanoreceptors are responsive
to pressure and especially to vibrations.
| I prefer to call these sensory organs Pacinian-Vater corpuscles instead of just Pacinian. The reason is a little historical story to round out your liberal arts dental education. Fillippo Pacini was an Italian histologist of the middle 1800's. Vater was his student. Vater discovered the onion shaped corpuscles but, of course, they were named after the professor. The least we can do is call them Pacinian-Vater corpuscles. After all, students do deserve a tensy wensy bit of respect, despite what you think. |
If you have trouble finding examples on your slide look for them on D-163, an optional extra slide of a Fingertip stained with H&E.
D-162 Scalp (H&E)
This section shows an example of hairy, thin skin. Being from the scalp it has numerous hair follicles, sebaceous glands and eccrine sweat glands. These are all ectodermal derivatives formed by invagination of the stratified epithelium into the underlying connective tissue. Sebaceous glands lie in the dermis. Sweat glands and the roots of the hairs extend deep into the dermis. The lowest layer of tissue (at the torn surface) is somewhat looser connective tissue with many holes in it. These spaces are artifacts formed when the technician pulled the scalp away from the underlying tissue. Push on your scalp (or your neighbor's, if you are brave enough) to realize that it rests on a layer of looser connective tissue which allows a certain amount of mobility. However that amount is limited (compare this with the skin on the back of your hand - or your neighbor's if you happen to be holding hands.)
Compare the epidermis of this thin skin with your recollection of the thick skin on slide D-161. Only three cell layers are discernible in thin skin, although you may also see a few granulosa cells with keratohyalin granules.
You can see three types of cells besides keratinocytes in the Malpighian layer, and distinguish two of them, (illustration).
Melanocytes are located in the stratum germinativum and send invisible arms of cytoplasm between the surrounding keratinocytes. They are invader cells, originating from the neurocrest and make fewer desmosome connections with their neighbors than keratinocytes do. Therefore, when skin tissue shrinks during histological preparation a space usually opens up around the melanocyte. This artifact makes it easy to find them.
This specimen was taken from a dark-skinned individual, and you can see granules of melanin pigment in the lowest cells as a brownish color. Melanocytes synthesize this melanin and then transmit it in granules to the surrounding keratinocytes. Since only the cells in the lowest one or two layers of keratinocytes divide these are the only ones that need pigmentation to protect their DNA. Obviously the keratinocytes get rid of their melanin granules when they get pushed farther up in the strata spinosum.
A second cell type that seems to live in spaces too large for them in the stratum germnitivum are the enigmatic Merkel cells. They tend to cluster at the base of epidermal pegs. Do not bother trying to tell them from melanocytes. The function of Merkel cells is a bit unclear. The cells are associated with nonmyelinated nerve fibers, suggesting that they may be mechanoreceptors, but they also have granules suggesting a secretory function.
Langerhans cells are the third type of clearer staining cells. They are relatives of macrophages and invade the epithelium to collect antigens. Then they migrate to lymph nodes to tell your immune system to make your skin break out in a rash. Langerhans cells invade throughout the stratum spinosum so a clear cell above the basal cell layer can be predicted to be this type.
One objective of this slide is to show the hair follicle and its associated structures. Under low magnification scan around until you find a near cross-section (evidenced by being round or egg-shaped) of a hair follicle. The center of the follicle will either be empty (because the hair fell out) or contain a circular structure with brown granules in it (the actual hair). The layer of tissue immediately surrounding the hair is the inner root sheath. Towards the base of the hair follicle the inner root sheath is two or three cells thick (count the nuclei) (illustration). If you are high up in the follicle, these cells will be keratinized and appear as a homogeneous pale staining ring (illustration). In any case, the next band out will be five to six layers of fairly large, pale-staining cells. This is the outer root sheath. Immediately outside of its basal lamina is a condensed connective tissue sheath, sometimes called the glassy membrane.
The part of the hair which actually grows is the hair root. It consists of a bulb of actively dividing epithelial cells invaded by a papilla of connective tissue. The papilla carries a blood supply for the actively dividing cells around it and a nerve supply that makes the hair a sensitive sense organ. (Shut your eyes and have your partner move a single hair on your arm with a pencil to realize just how sensitive it is). The epithelial cells in contact with the connective tissue papilla (or with the basal lamina which surrounds it) are stem cells. As the dividing cells get pushed up off of the basal lamina they continue to divide for a time, then differentiate as they are pushed farther along and become keratinized. Both the hair itself and the internal root sheath grow from the bulb. In contrast, the cells of the outer root sheath are produced by the outermost layer of cells of the root sheath. As the cells of the outer root sheath divide they push the daughter cells inwards instead of upwards. Thus, the epithelia of the inner and outer root sheathes are oriented perpendicular to one another. The above layers also can be recognized in longitudinal sections of hair follicles, but not as easily as in cross sections.
Sebaceous glands can be located as clusters of very pale-staining foamy-looking cells in the dermis associated with the hair follicles. You looked at them yesterday. They form as outpocketings of the epithelium of the outer root sheath and empty their secretion into the crevice between the hair and the outer root sheath surrounding it. That is, they use the hair follicle for their duct. Sebaceous glands are simple branched acinar glands with a stratified epithelium. Their cells undergo holocrine secretion. The entire contents of the cell is shed as the oily exudate called sebum. A single layer of flattened basal cells along the basement membrane at the outer edge of the gland can divide. As the resulting cells get pushed into the interior, they fill with lipid droplets. This gives them a spongy appearance. Eventually, the cells cannibalize their intracellular organelles and lyse. The lysed cells are pushed as a goo into the short duct leading into the hair follicle. Try to find a gland sectioned to show the full height of the epithelium so that you can trace the stages that the individual cells go through as they mature (example).
Just below a sebaceous gland, you may see a condensed pink band of smooth muscle with many elongated nuclei. This is the arrector pili muscle (high mag). It is anchored at one end to the basal part of the hair follicle and at the other end to the upper part of the dermis. Its function is obvious. Contraction raises the hair (to give you goose pimples!) Also, it runs right over the sebaceous gland so that it squeezes oily sebum out onto the surface of the skin as it erects the hair.
Finally, check out the sweat glands located near the sebaceous glands if you were unhappy with the way they were preserved in slide D-61. In fact, look at them again anyway. They are pretty neat things with their myoepithelial cells and unusual stratified cuboidal epithelium.
D-176 External ear canal (H&E)
The external ear canal is lined by thin skin with a few small hairs and associated sebaceous glands. The special structures to observe are the ceruminous glands, (illustration). These coiled tubular glands are modifications of apocrine sweat glands, to produce earwax. They are much larger in diameter than eccrine sweat glands. Their secretory cells are cuboidal and are said to contain brown pigment granules and fat vacuoles (but don't fret if you cannot find these cellular components). They are surrounded by a well developed network of myoepithelial cells. If you did not see this cell type in other glands be sure to notice them here. They are cells of the epithelium (ectoderm) that lie inside the basement membrane but are differentiated into multi-armed contractile cells. The ducts (high mag.) of the ceruminous glands have large lumens surrounded by a narrow layer of very flattened stratified squamous epithelium quite distinct from the secretory portion (illustration). You may be able to find a transition between the secretory and duct portions if you scan around carefully.
The main difference in appearance between ceruminous glands and generalized apocrine sweat glands of arm pits, groins and around nipples is that their ducts are larger in diameter, as is necessary to convey their much more viscous secretion.
D-7 skin with elastic stain
You have already looked at slide D-7 to examine the elastica of connective tissue. It shows that the dermis has a substantial component of elastica as well as collagen (red fibers). It also shows particularly clearly the differences between the papillary and reticular layers of skin, (illustration). The coarseness of the fibers immediately differentiates the loose connective tissue just below the epidermis and the very dense connective tissue making up the lower part of the dermis. It should not be too difficult to figure out why each of these layers has the type of connective tissue that it has.
Can you find blood vessels in the two layers of the dermis? If so, how do they differ in the two locations? Now examine the hypodermis. How does it differ from the reticular layer of the dermis?
* * * * *
Optional slides
D-164 Toenail (H&E)
Turn this slide around so that the label is on your left before you place it on your microscope stage. Under low power, find the stratified squamous epithelium on the top of the toe. Note its four layers. Follow along the epithelium until it turns under at the eponychium (cuticle), thins out to essentially the stratum germinativum, and turns back on itself deep in the dermis, (illustration). This area is the nail matrix. At this point, surface cells are becoming keratinized and collect into the nail root. This is the structureless light yellow area within the fold of stratum germinativum. Now follow the stratum germinativum out under the body of the nail and observe the longitudinal ridged appearance of this layer of the epidermis. This layer of epithelium that the nail slides along as it grows is the nail bed. Finally, as the free edge of the nail is reached, the stratum germinativum picks up the other epidermal layers, especially the stratum corneum, which is here called the hyponychium.
The skin beyond the hyponychium is another good example of thick skin. In it you can look for sweat glands and Meissner's corpuscles.
Figure out exactly where the dividing cells are that produce
the top of the keratin of the nail and the bottom. What direction do the
newly formed cells move? Draw the pathway from where the cells are born to where
they are lost. Both a hair and a nail develop from an invagination of the epidermis
into the underlying connective tissue. What layer of the nail would correspond
to the outer root sheath of a hair?
D-166 fingertip (silver)
This section of a fingertip has been stained with silver to show nerve fibers associated with sensory nerve endings. A plexus of minute nerve endings invade between the stacked cells. of the Meissner's corpuscle. These endings join to form a large, exiting, myelinated, sensory nerve fiber, which the supermen and women of the class may see partially. Frankly, I could not find an example worth photographing. You can have a bit more luck with Pacinian -Vater corpuscles. As mentioned earlier, a large myelinated nerve fiber pierces the corpuscle at one pole and continues through the core as a straight, unbranched and unmyelinated nerve fiber. It shows up in some cases as a black dot in the middle of the corpuscle.
D-163 thin skin of monkey (H&E)
This slide has a section of thin skin from an area other than the scalp. The epidermis is similar to that on your scalp slide D-162, except that it gives rise to fewer and smaller hairs. Observe the thickness of the strata, especially of the stratum corneum, which defines the skin as thin.
Under the epidermis the papillary layer of loose connective tissue grades into a denser reticular layer with coarser bundles of collagen. You may see recognizable examples of Meissner's corpuscles in the papillary layer.
Farther down the tissue is less satisfactory. There is an abundance of sweat glands but they are too poorly preserved to warrant study. In the lower part of the section areas of dense connective tissue intermingle with areas of highly cellular connective tissue. What type of cells do you think they are? Here is my guess for you to read after you have made a guess with your partners.
Your slide may have been cut from a different block of tissue. If so, do not worry about it. Instead, look to see what it has to offer.
D-165 Fingertip (H&E)
This slide is very much a repeat of slide D-161. If you found all of the structures described for that slide there is no particular reason to look at D-165. If not, use this slide. It is especially good for showing sensory nerve endings. Your slide should look like the picture above. There are sections from several other blocks in some of the student slide sets and some of these have poorly preserved sweat glands, sensory nerve endings and so forth.